The present invention relates to a cellular communications system, and more particularly to the selection of a transceiver site in a cellular communications system.
Cellular communications systems, such as those defined by the AMPS and GSM standards, are well known in the art. Such systems are characterized by the division of a geographical area into a number of cell sites, each served by a radio base station (RBS). FIG. 1. illustrates a portion of one such system. Any mobile station (MS) located in one of the cells may be in radio communication with the RBS, which in turn is communicatively coupled to a mobile system switching center (MSC) by, for example, an electrical or optical fiber cable. The MSC may establish connections between several MSs located within the cellular communications system and may also establish connections to entities outside the system by means of, for example, a link with a public switched telephone network (PSTN). These concepts are well known, and need not be described here in greater detail.
In order to avoid radio interference between nearby cells (called "cochannel interference"), such systems are designed to avoid using the same radio channels in nearby cells. However, the same channel may be used by different cells if they are sufficiently far apart from one another so as not to significantly interfere with each other. The distance between two such cells is called the "reuse distance."
Traditionally, cellular systems have been designed for telephones that are installed in cars. Each cell of such a system covers an area of approximately one square kilometer or more. The enormous popularity of cellular telephony and the introduction of hand held portable cellular telephones has now increased the demand for capacity of the system and for radio coverage indoors, and in other areas accessible to pedestrians.
As a solution to the problem of providing increased traffic handling capacity in cellular communications systems, smaller cells, referred to as micro or pico cells, have been provided. These smaller cells permit channels to be reused with greater frequency (i.e., with a smaller reuse distance), thus providing a mechanism for permitting the system as a whole to carry more traffic.
Micro/pico cells have been used to provide system coverage both outdoors and indoors. For example, it is advantageous to locate micro or pico cells in tunnels and the like. The fact that many small cells are used increases the cell planning complexity. The transceiver antennas must, in order to limit the cell size, be mounted close to the ground. However, this causes the radio propagation to be affected by stationary and movable obstacles, such as buildings, walls, doors and trees.
Another problem that occurs when designing a system to include small cells, is knowing exactly where the subscribers are most likely to be located while using their cellular telephones. Installing cellular radio equipment entails a substantial cost associated with the cellular radio equipment, the provision of electric and transmission facilities, etc. Therefore, it is of great economic interest to make an optimal choice when selecting the site for the base transceiver antenna. The site location must be optimal both with respect to radio propagation aspects as well as to traffic demand aspects (i.e., to ensure that the base transceiver antenna is situated at a location that will permit it to handle the largest number of calls). This is conventionally performed by making more or less clever guesses. However, due to the micro/pico cell's much smaller coverage area when compared to that of a "regular" cell, its performance is much more sensitive to the location of the base transceiver antenna. Thus, an error in determining a location for this antenna may have a substantial impact on micro/pico cell performance.
Various types of cellular communications system test equipment are known, but most do not address the problem of finding the optimal location of a base station site that will maximize the number of calls that will be handled by the new cell site. For example, the European patent document EP, A2, 0431956 discloses a system and method of evaluating the radio coverage of a geographic area serviced by a digital cellular radio telephone communications system. During operation, the position of at least one mobile subscriber within the geographic area is located when a call is received by a base station. The base station monitors the signal quality of the call and collects information relevant to the performance of the communications system. Location data and corresponding signal quality data are passed from the base station to an evaluating tool within the operation and maintenance center, where the characteristics of the radio coverage is presented. With this information, the system operator can diagnose coverage deficiencies and take the necessary corrective action.
A PCT application, WO, A1, 94/00932 describes devices for testing the voice transmission quality in a cellular communications system. A test set, programmed to monitor voice channels, is located within a cell site where it calls a responder connected to the switch of the mobile telephone switching office (MSTO) that serves the cell. A local switch with a responder at the base station may be provided to facilitate pinpointing any problems between the cell station trunk line and the wireless communication path.
Another PCT application, WO, A1, 91/15904 discloses base station transceiver equipment, for use in a cellular radio telephone system, that measures a parameter (e.g., timing, signal strength) of a mobile transceiver and instructs the mobile to adjust that parameter of its signals. A diagnostic subsystem or test mobile is provided in proximity to the base station transceiver equipment. The test mobile can change the parameter of its transmissions to simulate relative distance from the base station transceiver equipment for testing that the base station equipment issues the correct command.
An abstract of a patent document from the former Soviet Union, SU, A, 1185-626, outlines a radio station tester for monitoring the transmitted power level from a radio transmitter. The tester can be used for checking the response of a moving radio station to a calibrated level.
Yet another PCT application, WO, A2, 92,07429 describes an air interface monitoring unit with the capability of recording various messages and operational parameters. A unit having similar features is described in D. Yekta and F. Howat, "Product Review: Smith Myers CSM-8800 Cellular Service Monitor" Cellular Business, December 1993. However, the data collected by this unit does not tell a user how many MSs would utilize a hypothesized new micro/pico cell situated at the location of the air interface monitoring unit, because the unit does not discriminate between data collected from MSs that are close to the unit, and data collected from MSs that are far away from the unit. For purposes of planning a new micro/pico cell site, it is important to know how many MSs are in close enough vicinity to actually use the proposed site.
An advertisement for a Smith Myers Cellular System Simulator CSS2100 describes a unit that has the ability to simulate a cellular base station. That is, the unit can transmit signals to mobile cellular phones, whereby the mobile responds as if connected to the real cellular system and will obey and valid commands the unit sends. In particular, the CSS2100 can request all mobiles on a particular network in the immediate area to register, giving the user the number of mobiles in the immediate area. According to the advertisement, this unit is suitable for investigating the number of subscribers in a particular geographic area for the purpose of helping to decide as to whether another cell site is required in that area.
The CSS2100 has the drawback, however, in that by actively transmitting "dummy" control signals to MSs in order to get them to register, it may cause interference with the normal functioning of that cell.
Thus, there is the need to provide a mechanism for passively determining whether a proposed cell-site is the best for placement of a new base transceiver antenna.